1. Field of the Invention
This invention relates to fuel ignition systems, and more particularly to a control circuit for providing fail-safe operation of fuel valves of such systems.
2. Description of the Prior Art
In fuel ignition systems of the pilot ignition type, a pilot valve is operated at the start of an operating cycle to supply fuel to a pilot outlet for ignition to provide a pilot flame. When the pilot flame is established, a flame sensing circuit energizes a main valve to supply fuel to a main burner for ignition by the pilot flame typically, by operating a flame relay which closes contacts to connect power to the main solenoid.
Fail-safe control arrangements have been proposed in the prior art for preventing energization of the main valve for a fault condition of the flame sensing circuit which permits the flame relay to be operated in the absence of a pilot flame, or for a welded contact failure of the flame relay. These arrangements include a checking relay which when operated closes contacts which are connected in the energizing path for the main valve, such path being completed by contacts of the flame relay which is operated when a flame is sensed. The checking relay is energized over a path including normally closed contacts of the flame relay, and thus can be operated only if the flame relay is deenergized and its contacts are closed at the start of the operating cycle.
While fuel ignition control arrangements employing the checking relay function afford a high degree of fail-safe operation, the additional relay increases the cost of the control circuit. A further consideration is that the use of relays in the control circuit results in a larger package, making installation more difficult. That is, when the control circuit is used to control the fuel valves of a furnace in a heating system. The control circuit package, including the relays and the electronic control circuitry, is frequently mounted on the valve, and because of space limitations in the furnace vestibules, the control current package has to be disconnected from the valve while the valve is connected to the piping.
Although most known fuel ignition control circuits include relays, solid state ignition control circuits have been proposed previously. For example, in the U.S. Pat. No. 3,610,790 issued to A. W. Lindberg on Oct. 5, 1971, there is closed a solid state control circuit for a direct ignition system. The patented control circuit employs an SCR device which must assume conducting and then nonconducting states in effecting valve operation. At the start of an operating cycle, a pulse generating circuit provides trigger pulses for enabling the SCR device to conduct and energize the main valve solenoid connected in series with the SCR device. When the fuel is ignited and a flame is sensed, a flame sensing circuit inhibits the pulse generating circuit, terminating trigger pulse generation. In the absence of trigger pulses, the SCR device is rendered nonconducting, interrupting the energizing path for the main valve solenoid. The main valve solenoid is maintained energized over a holding path provided by a resistance which is shunted by the SCR device when it is conducting.
The flame responsive turnoff of the SCR device is achieved by leaking the charge off of a capacitor using the spark electrodes which are bridged by the main burner flame when the fuel is ignited. Thus, a resistance across the spark electrodes could also leak the charge off the capacitor an simulate a flame permitting the main valve to be operated in the absence of a flame.